1. Field of the Invention
The present invention relates generally to wide bandwidth compensating circuits for compensating for the nonlinearity of other circuits placed in cascade therewith over a relatively wide bandwidth, and more particularly pertains to such wide bandwidth compensating circuits for providing high linearity optical modulators, such as Mach-Zehnder interferometer modulators. In the compensating circuit, first and second versions of a signal are generated having different gains and different nonlinear distortions. The first version signal has a higher gain with lower distortion. The second version signal has a lower gain with higher distortion, and is combined with the first signal to form a resultant signal which effectively cancels the nonlinearity of a circuit placed in cascade therewith.
2. Discussion of the Prior Art
Circuits for canceling distortion often operate well only over a relatively limited narrow bandwidth, primarily because of reactive components therein. This causes the distortion to become frequency dependent.
A popular prior art approach for designing a circuit with a predetermined nonlinearity transfer characteristic uses a plurality of diodes, each of which connects a signal node to a voltage source through a predetermined resistance. As the signal voltage changes, and the diodes transition from reverse to forward bias (or vice versa), the effective impedance of the signal node changes, thereby producing the desired nonlinearity.
Accordingly, it is a primary object of the present invention to provide wide bandwidth compensating circuits with predetermined nonlinear characteristics to compensate for the nonlinearity of circuits placed in cascade therewith, particularly to achieve high linearity optical modulators.
A further object of the subject invention is the provision of such wide bandwidth compensating circuits in which first and second versions of a signal are generated having different gains and different nonlinear distortions. The first signal has higher gain with lower distortion. The second signal has lower gain with a higher distortion, and the first and second signals are combined to form a resultant signal having a predetermined nonlinear characteristic. The combination of the nonlinear characteristic of the compensating circuit and the nonlinear characteristic of the circuit in cascade produces a combined substantially linear response over a wide bandwidth.